Japanese Patent Laid-Open No. 2004-840 describes a conventional water treatment technique.
Referring to FIG. 6 of Japanese Patent Laid-Open No. 2004-840, sludge and excrement 41 from water treatment tanks are guided to an aeration tank 42, where a biological treatment is carried out on the sludge and excrement 41 using microorganisms. The biologically treated water is fed via a biological treatment tank 43 to a first membrane separation apparatus 44, which separates the sludge and excrement into concentrated sludge 45 and membrane separated water 46.
The concentrated sludge 45 is partly returned to the aeration tank 42 and the biological treatment tank 43 as sludge to be returned. The rest of concentrated sludge is continuously or intermittently fed to a dehydrator 47. On the other hand, the membrane separated water 46 is mixed with a flocculation agent in a mixture tank 48. The mixture is then guided to a flocculation tank 49, in which flocs are formed. The resulting water is then guided to a flocculation membrane raw water tank 50 and then to a second membrane separation apparatus 51. In the second membrane separation apparatus 51, the water is separated into flocculated concentrated sludge 52 and membrane separated water 53 with a flocculation system.
The membrane separated water 53 with a flocculation system is guided out of the system, and the flocculated concentrated sludge 52 is partly returned to the flocculation membrane raw water tank 50, with the remaining sludge continuously supplied to the dehydrator 47 as sludge slurry together with the rest of concentrated sludge 45. The dehydrated sludge 55 is guided out of the system, and a dehydrated separated liquid 56 separated from the sludge is continuously returned to the biological treatment water tank 43.
Another conventional technique is disclosed in Japanese Patent Laid-Open No. 2003-236584. The technique will be described with reference to FIG. 7.
A sewage treatment apparatus 30 comprises a pretreatment facility 31, a first flocculation separation facility 32, a biological treatment facility 33, a second flocculation separation facility 34, an oxidization facility 35, an adsorption facility 36, a demineralization facility 37, a drying facility 38, and an effluent facility 39.
The pretreatment facility 31 carries out a pretreatment such as adjustment and homogenization of the amount and quality of sewage. The first flocculation separation facility 32 carries out a flocculation precipitation separation treatment and is located downstream of and adjacent to the pretreatment facility 31. The biological treatment facility 33 carries out a biological treatment and is located downstream of and adjacent to the first flocculation separation facility 32. The second flocculation separation facility 34 carries out a flocculation membrane separation treatment and is located downstream of and adjacent to the biological treatment facility 33.
The oxidization facility 35 carries out an advanced oxidization treatment and is located downstream of and adjacent to the second flocculation separation facility 34. The adsorption facility 36 carries out a suction treatment and is located downstream of and adjacent to the oxidization facility 35. The demineralization facility 37 carries out a demineralization treatment and is located downstream of and adjacent to the adsorption facility 36.
The drying facility 38 takes out solid salt and is located downstream of and adjacent to the demineralization facility 37. The effluent facility 39 releases the final treated water to a public water area and is located downstream of and adjacent to the demineralization facility 37.
As described above, in Japanese Patent Laid-Open No. 2004-840, the biologically treated water biologically treated in the aeration tank 42 is subjected to membrane separation at multiple levels by the first membrane separation apparatus 44 and the second membrane separation apparatus 51. In particular, in the second membrane separation apparatus 51, the flocculation agent is used together for flocculation membrane separation.
Japanese Patent Laid-Open No. 2003-236584 achieves an advanced treatment by carrying out the flocculation, precipitation, and separation treatment in the first flocculation separation facility 32, the biological treatment in the biological treatment facility 33, and the membrane filtration separation treatment with a flocculation agent in the second flocculation separation facility 34.
However, the BOD concentration of the biologically treated water is not constant but varies depending on the BOD concentration of raw water that has not been biologically treated. Thus, when an advanced treatment is carried out in a reaction tank with a membrane separation apparatus placed therein, using a flocculation agent together, the following problems may occur.
The low BOD concentration (for example, less than 20 mg/L) of the biologically treated water reduces the amount of sludge generated in the reaction tank (the amount of microorganisms) and thus the concentration of organic substances in the reaction tank decreases. Thus, even with the addition of the flocculation agent, smaller sludge flocs are formed in the reaction tank, and said sludge flocs are likely to be dispersed easily, so that the amount of flocculation agent might be increased.
An environment with a lower inflowing BOD concentration causes the exhaustion of extracellular substrates of the microorganisms in the reaction tank. The microorganisms thus start to use intracellular carbon sources (endogenous substrates), causing the self-degradation of microorganisms contained in the sludge flocs, which are thus dispersed in the form of fragments. Thus, the dispersed very small fragments of the sludge and microorganisms self-degrade into small solid substance of the microorganisms. This makes a membrane surface in the membrane separation apparatus likely to be occluded.
When a submerged membrane separation tank with a flocculation system is used for the advanced treatment, a certain range of sludge concentration is suitable for operation. This sludge concentration range is required to ensure sufficient treatment performance or to form sludge flocs of a preferred size. However, a low inflowing BOD concentration requires a long time to reach the sludge concentration suitable for operation or makes it difficult to reach that concentration.
The present invention overcomes these and other problems, and provides a water treatment system using a membrane separation apparatus to treat biologically treated water, the system making it possible to inhibit fouling of membrane surfaces and to reduce the amount of flocculation agent used.